1. Field of the Invention
The present invention relates generally to micro electro-mechanical systems (MEMS), and more particularly but not exclusively to MEMS light modulators.
2. Description of the Background Art
MEMS devices typically include micro mechanical structures that may be actuated using electrical signals. MEMS devices may be employed in various applications including light modulation for printing, video display, optical networks, and maskless lithography. Example MEMS light modulators include the Grating Light Valve™ (GLV™) light modulators available from Silicon Light Machines, Inc. of Sunnyvale, Calif. (Grating Light Valve™ and GLV™ are trademarks of Silicon Light Machines). A light modulator may employ movable ribbon-like structures. A ribbon may be deflected to modulate light incident thereon.
FIG. 1A schematically shows a top view of a portion of a conventional ribbon-type diffractive spatial light modulator 100. Light modulator 100 includes ribbon pairs 110, with each ribbon pair 110 consisting of a deflectable active ribbon 112A and a stationary bias ribbon 112B. In some applications, 3 ribbon pairs 110 are employed to represent one pixel of information (e.g. a pixel of a video image). The ribbons 112 (i.e. 112A and 112B) are symmetrical about a symmetry line 102. The right hand portion of the ribbons 112 are not shown for clarity of illustration. In operation, a light source illuminates the optically active area 114 of the ribbons 112. The optically active area is also referred to as a “sweet spot” as it is the portion of the ribbons 112 configured to be illuminated by a light source. In the example of FIG. 1A, active ribbons 112A are configured to deflect, while bias ribbons 112B are configured to remain relatively stationary or fixed. Light modulator 100 represents a particular implementation where the ribbons are used as modulator elements, as opposed to embodiments wherein the ribbons and the gaps between the ribbons are used as the modulating elements.
FIG. 1B schematically shows a side cross-sectional view of the light modulator 100 taken at section A—A of FIG. 1A. Each ribbon 112 (i.e. 112A or 112B) comprises a reflective material 120 supported by a resilient structure 121. A gap separates the ribbons 112 from the substrate 122. On top of the substrate is a drive electrode 131, also referred to as a “bottom electrode.” The reflective materials 120 may be configured as actuator electrodes, also referred to as “top electrodes.” Applying a potential difference between the drive electrode 131 and the actuator electrodes creates an electrostatic force that deflects the actuator electrodes toward the substrate 122.
FIG. 1C schematically shows the ribbons 112 of FIG. 1B when the active ribbons 112A are actuated. As shown in FIG. 1C, a height difference between adjacent ribbons can be changed by controllably deflecting the active ribbons 112A towards the substrate 122 by up to about 9λ/4 and more typically about 5λ/4, where λ is the wavelength of the incident light. If, upon reflection, the light from adjacent ribbons is in phase, then the 0th order light reflection is effectively maximized and the light modulator 100 is in an ON state. To minimize the 0th order light reflection; the active ribbons 112A are deflected by an odd multiple of the wavelength. When the 0th order light reflection is minimized, the light modulator 100 is in an OFF state. The ribbons 112A may be actuated such that the light modulator 100 is ON, OFF, or in between to modulate incident light.
The speed of currently available devices employing ribbon-type diffractive spatial light modulators is limited by damping time “T” (tau), which is the time required for a ribbon to transition from an OFF state to an ON state, or from a first deflected state to an undeflected or a second deflected state. FIG. 2 shows a graph 200 illustrating the impact of damping time on transition from an OFF state to an ON state in a conventional ribbon-type spatial light modulator, such as light modulator 100. Plot 204 shows a simulated response of a conventional ribbon-type spatial light modulator having a response time of about 4 microseconds, while plot 202 shows the simulated response of a conventional ribbon-type spatial light modulator having a response time of about 6 microseconds. As shown in FIG. 2, the transition from OFF to ON (region I to II) results in oscillation such that the integrated intensity is lower in region II than in region III for both plots 204 and 202. Thus, the minimum pulse time for the transition (i.e. the maximum speed of the device) is limited by the maximum allowable variation.
FIG. 3 shows a diagram and a formula illustrating the impact of various characteristics of a ribbon-type diffractive spatial light modulator on damping time. The factors that affect damping time include: gap thickness (G) that is the distance separating a lower surface of the ribbon from an upper surface of the supporting substrate; ribbon density (ρ); ribbon thickness (t); ribbon width (w); and the gas effective viscosity (ηeff) of gas enveloping the light modulator and filling spaces between the ribbons and substrate. As the gas becomes more viscous, the damping time is increased. Although many, if not all, of these factors can be optimized for speed (i.e. to minimize damping time), there is typically a compromise of other device performance parameters including wavelength of modulated light, illumination efficiency or fill-factor, diffraction angle, die or modulator size, sweet spot size, snap-down margin, or operating voltages. FIG. 4 shows a chart illustrating the tradeoff between optimization of speed realized through decreased damping time and spatial light modulator performance in a conventional ribbon type spatial light modulator.